Laser projectors are widely used in manufacturing processes to assist in precision assembly of large scale structures, composite articles, etc. in aerospace, construction and other industries. Laser projectors are distinguished from digitizing scanners. U.S. Pat. No. 6,246,468 to Dimsdale is one example of a laser scanner that uses pulsed laser light to determine range to points on an object and create a point cloud of image data points In the Dimsdale system, a separate video system gathers information about the intensity of the reflected light.
Known laser projectors use a scanned output beam of a continuous wave laser to generate glowing templates on a 3D object surface utilizing computer assisted design (CAD) data for projection trajectories. Typically laser projectors include optical feedback to assist in defining projector's location and orientation in 3D space with respect to the object's coordinate system. This defining is commonly termed “bucking in.” It requires use of several, typically three to six, reference (fiducial) points selected or placed on or about the work surface of the object. One specific example of this type of laser projector, for example, is disclosed in U.S. Pat. No. 5,450,147 to Palmateer. The '147 laser projector system uses a plurality of cooperative reference targets mounted, on or adjacent to, the object. These targets return the laser light back into the projector's beam steering system. Another laser projector disclosed in U.S. Pat. No. 5,381,258 to Bordignon specifically requires reference targets to be retro-reflective. Yet another laser projector described in Kaufman and Savikovsky U.S. Pat. No. 6,547,397 issued to two of the present inventors relies on reference targets for both distance ranging and angle measurement.
The requirement to place reference targets onto the object has many practical drawbacks to the process of using laser projectors. It is time and labor consuming. It also degrades precision and reliability due to a lack of precision in the placement and resultant position of the target. Some potentially viable applications currently cannot be implemented because they do not allow any target placement on the object surface.
The main reason retro-reflective reference targets are used in almost all laser projecting systems is because they provide quite distinguishable optical feedback signal by returning a substantial portion of projected laser light back into the beam path through the beam steering system.
The maximum output laser beam power allowed for laser projectors due to laser safety regulations is 5 milliwatts. The power of the portion of the laser light that is reflected from a typical retro-reflective target and directed back through the beam steering system is typically about 200 to 1,000 nanowatts depending on the distance between projector and a target and on the size of the beam steering mirrors.
A number of solutions are proposed in the prior art to deal with the problem of the optical feedback using the same beam path through the beam steering system as the output projector beam. They involve different ways to separate the output laser beam from the received feedback light in the laser projector. The aforementioned Palmateer '147 patent utilizes a beam splitter. The Bordignon '258 patent teaches using a particular wedge-shaped lens with a central opening for the output beam. Laser projectors in Kaufman and Savikovsky '397 patent use a reflective optical pick-up prism. Each of these solutions provides somewhat different effectiveness of utilizing received feedback light that is directed toward a photo detector. Using retro-reflective targets and these known solutions to the problems of a shared optical path, typical optical feedback beams that reach the photo detector are estimated at 50 to 500 nanowatts of power.
It is very desirable in laser projection to use the object features (e.g., corners, holes, fasteners, etc.) as fiducial points for laser projection instead of separately placed retro-reflective targets. However, prior attempts to solve this problem have not provided a solution without other drawbacks. For example, U.S. Pat. No. 5,615,013 to Rueb offers a solution combining a galvanometer and a camera system. A serious drawback of the Rueb arrangement is the existence of two different optical paths for laser projection and camera imaging, which necessitates for frequent mutual calibration between the camera imaging system and the laser projection system. It is necessary to use separate reference targets in the process of this mutual calibration. As a result, the suggested solution reduced accuracy.
In order to maintain a high level of laser projection precision (e.g. to within ±0.015 inch at a laser-to-object distance of 15 feet), it is required that the beam path through the beam steering system is the same for both the optical feedback and the output projector beam. However, if retro-reflective targets are not used, the power level of light diffusely reflected back from a typical object material like plastic or painted metal, and returned through the projector beam steering system, has been determined to be about 1,000 times less than the reflected light power from a typical retro-reflective target. That means the typical optical feedback beam that reaches a photo detector is roughly in the range of 50 to 500 picowatts of power. In other words, the typical optical feedback beam power from the non-target object feature that reaches the photo detector is about 100 million times less than the output laser projector beam power. Because the output beam has to share the optical path with the feedback beam it adds prevailing, unwanted background light due to the light scatter and secondary reflections. This unwanted “stray” light renders the optical feedback signal undistinguishable.
In a conventional laser projection application for product assembly, once all the known fiducial points have been detected, a laser projector's computer runs mathematical algorithm to calculate precise position and orientation of the laser projector with respect to the object. Then it starts actual projection. It generates a series of beam steering commands in a precisely arranged way to direct the beam at each given moment of time exactly toward the given trajectory CAD point (x, y, z) on the surface of the 3D object. The beam strikes the surface of the object following the computer-controlled trajectory in a repetitive manner. With sufficiently high beam speed, the trajectory of the projected beam on the object's surface appears to human eye as a continuous glowing line.
Glowing templates generated by laser projection are used in production assembly processes to assist in the precise positioning of parts, components, and the like on any flat or curvilinear surfaces. Presently laser projection technology is widely used in manufacturing of composite parts, in aircraft and marine industries, or other large machinery assembly processes, truss building, and other applications. It gives the user ability to eliminate expensive hard tools, jigs, templates, and fixtures. It also brings flexibility and full CAD compatibility into the assembly process.
In the laser assisted assembly process, a user positions component parts by aligning some features (edges, corners, etc.) of a part with the glowing template. After the part positioning is completed, the user fixes the part with respect to the article being assembled. The person assembling the article uses his or her eyesight to make a judgment about proper alignment of the part to the glowing template. Because this process relies on the visual judgment of a worker, it is subjective, and its quality may be substantially reduced by human errors.
Human errors adversely impact any manufacturing process, they are unacceptable, and they have to be revealed as soon as possible. In aircraft manufacturing, for example, every production step has to be verified and properly documented. One hundred percent quality assurance is often required. Therefore, a device and method that combines the capabilities of laser projection with immediate verification of part placement during assembly process are very desirable. They would provide the benefits of revealing and fixing human errors right on the spot, thus avoiding very costly and time-consuming off-line testing procedures.
Existing 3D laser projectors use design model CAD data to generate glowing templates on a 3D object surface. Hence, precision of the laser projection is adequate only if the object is built exactly up to its model.
There is therefore a great need for an effective way to deliver an optimized 3D laser projection on the surface of the object that differs from its design model. For example, U.S. Patent Application No. 2005/0121422 A1 discloses a technical solution based on sequential use of a separate 3D digitizing system, first, to determine as-built condition of the object, then, second, to modify as-design data for laser projection utilizing results of digitizing, and then, third, using a separate 3D laser projector to generate a glowing template. This system is complicated, requires use of a cage with retro-reflective targets and requires sequential usage of two separate expensive units of equipment—digitizer and projector. Because of the need to use multiple units and multiple operations, this system is limited in its accuracy, particularly the high precision required for modern aircraft and other precision manufacturing, e.g. to ±0.015 inch at 15 feet.
Another example—an industrial process for an aircraft composite component diagnostics/repair—is disclosed in the article “Update: Automated Repair” in High-Performance Composites, March 2007, pp. 34-36. Again, this solution requires separate sequential use of two very different and expensive units of equipment—3D digitizing system and 3D laser projector.
Hence, there is a need for a laser projector with full 3D digitizing capabilities, including ability to detect object features.
Known solutions for standalone 3D scanning digitizers are not suitable for laser projection with feature detection. For example, the Laser Radar LR200 manufactured by Metris utilizes a large gimbal-mounted mirror for beam steering and an infrared laser for distance measurement. This system, however, is quite slow and it cannot generate glowing templates as a laser projector.
A 3D scanning digitizer with a time-of-flight ranging system is disclosed as one embodiment in U.S. Pat. No. 7,215,430. It utilizes a pulsed “green” laser with a relatively low repetition rate, an avalanche photodiode as a detector, and a threshold-based timing circuit. This solution relies on separate video cameras to obtain an image of the object. It cannot provide required accuracy for the high precision edge features detection. Also, the described system does not capture signals returned from objects well if they have a substantially variable reflectivity. For such objects this system obtains very sparse data with low spatial resolution. Again, the disclosed apparatus is only a digitizer; it cannot effectively generate glowing templates for laser projection.
The dynamic range of a digitizer in dealing with variations in the intensity of light returned from a scanned object is a serious practical limitation on the usefulness of known digitizers. Scanning a white wall versus scanning a black wall can produce 100× variations in the reflected light intensity. A white wall scanned at a distance of six feet produced a variation of roughly 3,000× compared to a black object at 40 feet distance. A polished steel ball is a particularly difficult object to scan because it produces basically a strong single point reflection surrounded by much weaker reflections. A versatile 3D digitizing scanner and projector should have a dynamic range of 100,000 to 500,000 to work with a wide variety of objects and in a wide variety of operating conditions encountered in practical applications. Known scanners and known projectors do not provide anywhere near this dynamic range.
A laser projector solution capable of distance ranging is described in the aforementioned Kaufman and Savikovsky U.S. Pat. No. 6,547,397. However, as noted above, it works only with retro-reflective reference targets (surfaces). Moreover, it does not provide adequate sensitivity and dynamic range to accurately acquire a variety of conventional surfaces and 3D objects and to detect their edge features. It projects a glowing template using a continuous wave laser. In ranging to orient the projector to a workpiece, it uses a pulsed laser beam, but one pulsed at the maximum rate near 1 kHz. Distance information for all pixels corresponding to the scanned pulses cannot be obtained.